Transmyocardial revascularization is a recently developed addition to the armamentarium of treatments for cardiovascular disease. Previous approaches to treatment of cardiovascular disease have relied on two basic strategies: rerouting blood flow around blockages in the coronary arteries (coronary artery bypass grafting), and reducing or removing the atherosclerotic plaque causing a stenosis or occlusion in the coronary arteries (balloon angioplasty, laser angioplasty, and atherectomy). While some of these techniques have been extremely successful and have provided tremendous benefit to a great many patients, some patients remain untreatable using these approaches. Very elderly patients and patients who are weakened by very severe cardiovascular disease or other concurrent disease conditions and patients who have undergone previous coronary bypass surgery are considered to be bad surgical risks for coronary artery bypass grafting. Some other patients are not treatable using catheter approaches, such as balloon angioplasty, laser angioplasty, or atherectomy, because of diffuse arterial disease or extremely tortuous coronary arteries or recurrent restenosis.
Transmyocardial revascularization (TMR) offers an alternative approach to treatment of cardiovascular disease in which blood flow passages are artificially created through the myocardium directly from the interior of the ventricular chambers. The first attempts at transmyocardial revascularization were performed on animal models using a needle to create passages through the cardiac wall from the epicardium to the interior of the ventricular chambers. The puncture site in the epicardium would quickly seal over, leaving an open blood flow passage from the ventricular chambers into the myocardium. However, the patency of the blood flow channels created was short lived because the needle punctures through the myocardium would eventually heal over, closing off the blood flow passages within about two weeks after treatment. The next approach to transmyocardial revascularization involved the use of a surgical CO.sub.2 laser to ablate blood flow passages from the exterior of the heart through the myocardium. This approach proved more successful because the laser-created blood flow passages through the myocardium did not heal over and therefore remained patent longterm. Perfusion studies, using radioactively labeled microspheres, demonstrated improved perfusion of the ischemic myocardium treated with TMR. A variation on this technique, described by Hardy in U.S. Pat. No. 4,658,817, uses a hollow needle to initially puncture the epicardium, then the laser energy is delivered through the needle lumen to ablate a passage through the myocardium into the ventricle. The theory behind this approach is that the needle punctures in the epicardium will heal quickly, while the laser-created blood flow passages through the myocardium will remain patent longterm. This patent and other U.S. patents referred to herein are hereby incorporated by reference in their entirety.
The disadvantage of these approaches to TMR is that the infrared beam of the surgical CO.sub.2 laser, which was needed to effectively ablate the myocardial tissue, must be delivered through an articulated-arm waveguide using metal coated reflectors to direct the laser beam. The use of the articulated-arm waveguide necessitated that the TMR procedure be performed as open-chest surgery, which carried with it patient risk and trauma almost equivalent to standard coronary bypass surgery. The development of near infrared lasers, such as the holmium:YAG laser, which were effective for tissue ablation and which could be delivered through specialized fiberoptic devices, created opportunities for less invasive and therefore less traumatic approaches to TMR. U.S. Pat. No. 5,380,316 granted to Aita et al. describes a method for intra-operative myocardial device revascularization which uses a flexible fiberoptic device to deliver a beam from a holmium:YAG laser to the exterior of the heart through only a small incision in the chest wall. U.S. Pat. No. 5,389,096, also granted to Aita et al., describes a system and method for percutaneous myocardial revascularization which uses a flexible fiberoptic device to deliver a beam from a holmium:YAG laser to the interior wall of the heart through a percutaneous transluminal intravascular approach.
One of the major advantages of the TMR technique is that the procedure can be performed on a beating heart. This is significant because the second most important source of morbidity and complications during coronary bypass surgery, after the trauma of the median sternotomy to open the chest, is the cardiopulmonary bypass system, or heart-lung machine, which is used to support the circulatory system during extended periods of cardioplegic arrest. Operating on the beating heart eliminates many of the complications and risks of using the cardiopulmonary bypass system. However, using a laser on the beating heart carries with it a number of other inherent difficulties as well. One disadvantage of performing TMR on a beating heart is that it is difficult to verify whether the laser beam has fully penetrated the ventricular wall so that the practitioner can be sure that each blood flow channel makes a fluid connection with the interior of the ventricular chamber. The contraction of the heart muscle prevents accurate probing of the laser-created channels to verify complete penetration. In addition, at certain times during the cardiac cycle, the heart is particularly sensitive to the laser beam. For example, if the laser beam strikes the heart during the T portion of the electrocardiogram (ECG) wave, heart fibrillation can occur, which, if it is not corrected immediately by cardioversion or defibrillation, can lead to heart failure and death. To avoid this, Aita et al. recommend monitoring the heartbeat and gating the laser so that it generates one or two pulses during contractions of the ventricle (systole) and to generate no pulses during the rest of the heart cycle. U.S. Pat. No. 5,125,926 granted to Rudko et al. describes a system for synchronizing the laser pulses with the beat of the heart while performing TMR in order to avoid inducing fibrillation.
Synchronization is only a partial solution, however, because the motion of the cardiac wall due to the beating of the heart causes difficulties in positioning and focusing the laser device. Consequently, even with synchronization, only a fraction of the 15 to 30 laser pulses typically applied during TMR result in a patent blood flow passage from the ventricular chamber into the myocardium. The result of this is that it prolongs the procedure, it reduces the effective revascularization of the ischemic myocardium, and at the same time, it unnecessarily increases the laser-induced trauma to the heart. These problems would be minimized if the heart were stopped during the procedure, but, as discussed above, previous approaches to this have brought with them the problems inherent with cardiopulmonary bypass systems.